U.S. Pat. 3,314,108 discloses an extruder where a rotatable rotor is placed inside the frame and a fixed mandrel is further placed inside the rotor, whereupon feed gaps are formed between the rotor and the frame and the rotor and the mandrel, respectively, the material to be extruded being supplied out of the extruder via the feed gaps by rotating the rotor provided with grooves. In such an extruder it is very difficult to control the temperature of the material to be extruded since it can only be performed either via the external frame or the internal mandrel. If the product to be extruded is to be provided with circumferential orientation, a separate orientation tool that is placed after the nozzle must be used in connection with the extruder in question and also with other typical extruders. This kind of an arrangement is disclosed for example in WO 95/24304. Adding such a separate orientation tool naturally makes the apparatus complicated and difficult to use. Further, for example when films are extruded it is necessary to use some other similar difficult rotating arrangement to compensate for the variation in thickness.
DE 1,961,078 discloses an extruder where a rotatable rotor is mounted in bearings inside the frame and where a conical stator is placed inside the rotor, whereupon between the rotor and the stator there is a conical feed gap along which the material to be extruded is pressed out of the nozzle of the apparatus by means of rotor grooves as the rotor rotates. In this case, too, it is difficult to control the temperature of the material to be extruded since the temperature can only be regulated for the part situated inside the feed gap. Further, cooling of the rotor is a considerable problem since the rotor is encased inside the frame. Also, the extruder requires a separate tool placed after the nozzle for the orientation or the compensation of variation in thickness. The structure of the extruder is altogether rather cumbersome and difficult.
The purpose of the present invention is to provide a method and an apparatus where the aforementioned problems can be avoided. Another purpose is to provide an extruded product with very good properties.